The tertiary and quaternary structure of oligomeric proteins participating in supra-molecular assemblages will be studied by means of x-ray diffraction techniques. In addition to the important biochemical information on the structure and function of the separate component proteins, it is hoped that these investigations will provide added insights into the nature of complex molecular organizations that are prevalent in all living organisms. We have crystallized the C-phycocyanin hexamer from the blue-green alga Agmenellum quadruplicatum. These crystals diffract beyond 2.5 A resolution and have only a monomer in the asymmetric unit of its hexagonal cell. The phycobilins, with their linear tetrapyrrole derivative chromophores, associate as ordered granules termed phycobilisomes, which serve as the light harvesting antennae for photosynthesis. There is evidence that the rods recognized in electron micrographs correspond to the stacks of phycocyanin hexamers that result from the crystal packing. Our low resolution x-ray studies could provide information on the organization of the phycobilisome, the subunit structure of the hexamer, and the distribution of its chromophores. The high resolution studies will allow us to study the conformation(s) of the chromophores and their interactions, compare the alpha and beta chains of the phycocyanin monomer, and study the various subunit-subunit interactions present in the assemblage. We are also in the preliminary stages of studying the component enzymes of the Escherichia coli pyruvate dehydrogenase complex which catalyzes the conversion of pyruvate to acetyl-CoA for entry into the tricarboxylic acid cycle. We have obtained crystals of both the transacetylase and flavoprotein components, and experiments are in progress to crystallize the pyruvate dehydrogenase component. Our present goal is to obtain sufficient low resolution-ray and electron microscopic data to determine the shape of the component enzymes and locate their active sites so that the entire complex can be modelled and the interaction of its active sites and its regulation studied.